Euclid's Elements - Book VII: Proposition 2.

Proposition 2. Δύο αριθμών δοθέντων μη πρώτων πρός αλλήλους το μέγιστον αυτών κοινόν μέτρον ευρείν. Heath: To find the greatest common measure of two given numbers (which are) not prime to one another. Proposition 2 uses the same obelus division or antenaresis as does Proposition 1, except that in the case of Proposition 2, no remainder implies a common measure greater than 1. Example: Find the greatest common denominator of 990 and 560. Always measure the greater (990) with the smaller (560): 990 = 560(1)+430 560=430(1)+130 430=130(3)+40 130=40(3)+10 40=10(4) One can also find the greatest common divisor by factorising each of the numbers and then picking out the product intersection: 2x2x2x2x5x7=560 2x5x3x3x11=495 2 x 5 = 10 Link to applet used in presentation: https://drive.google.com/file/d/14Zf-1DWuJhAkTmi420aDSq7xAFh9vmUu Link to New Math Channel: https://www.youtube.com/channel/UCRzKlAytuyb2qE8Oa44Wumw/videos